(display "\n========================================\n")
(define (lookup given-key set-of-records)
    (if (null? set-of-records)
        #f
        (let ((current-record (car set-of-records)))
            (let ((current-key (key current-record)))
                (cond   ((= given-key current-key) current-record)
                        ((< given-key current-key) #f)
                        ((> given-key current-key)
                            (lookup given-key (cdr set-of-records))))))))
; 没看清楚，用表实现。。。。下面用二叉树
; 假设有了 entry left-branch right-branch key 等过程
(define (lookup given-key tree-of-records)
    (if (null? tree-of-records)
        #f
        (let ((current-record (entry tree-of-records)))
            (let ((current-key (key current-record)))
                (cond   ((= given-key current-key) current-record)
                        ((< given-key current-key)
                            (lookup given-key (left-branch tree-of-records)))
                        ((> given-key current-key)
                            (lookup given-key (right-branch tree-of-records))))))))

(display "\n========================================\n")
